Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
In one example, the risk of pre-ignition may be reduced by lowering the thermal loading of a vehicle engine. One example approach for reducing engine thermal loading is shown by Ito et al. in EP1722090 A2. Therein, a liquid cooling system including a coolant jacket coupled to a cylinder head is used to dissipate large quantities of heat. Further, fuel enrichment is carried out for a duration during conditions when high exhaust gas temperatures are expected. The heating and evaporation of the excess fuel injected during the enrichment
However, the inventors herein have identified potential issues with such approach. As one example, the amount of heat dissipated by the cooling system may not be adequate to reduce the risk of engine thermal overloading, in particular, in the region where exhaust lines from different cylinders converge to form a common exhaust line. Thermal overloading issues may be exacerbated in boosted engines. As another example, the engine enrichment used to address thermal overloading may degrade fuel economy and exhaust emissions. Specifically, the enrichment may interfere with lean, or stoichiometric, exhaust conditions required for the operation of various exhaust emission control devices. Likewise, prolonged enrichment may lead to soot accumulation and coking of spark plugs, which in turn can generate misfires, and increase the propensity for pre-ignition. Soot may also deposit on the valves, hindering charge exchange and jeopardizing the sealing of the combustion chamber. That is, valves may actually be partially open when the valves are supposed to be fully closed.
Thus in one example, some of the above issues may be at least partly addressed by a method of operating an engine. In one example embodiment the method comprises, in response to an indication of pre-ignition, operating a first cylinder rich while operating a second cylinder lean for a first duration. The method further comprises, after the first duration, rich operating the second cylinder while lean operating the first cylinder for a second duration.
In this way, by using a combination of rich and lean operation in one or more engine cylinders thermal overloading of any given cylinder is reduced. Further, by alternating rich and lean operation of cylinders over a duration, issues related to prolonged cylinder enrichment or enleanment may be reduced.
In one example, during a first engine cycle, a first cylinder (or first group of cylinders) may be operated rich while a second cylinder (or second group of cylinders) is operated lean. Then, in the immediately following second engine cycle, the first cylinder may be operated lean while the second cylinder is operated rich. Likewise, in the immediately following third engine cycle, the first cylinder may be operated rich again while the second cylinder is operated lean again. The first and second cylinders may be selected and grouped based on their cylinder pre-ignition counts as well as their firing order. Further, in any given engine cycle, the enrichment of the rich-operated cylinder(s) may be adjusted to balance the enleanment of the lean-operated cylinder(s) to maintain the exhaust air-to-fuel ratio at stoichiometry.
In this way, a combination of enrichment and enleanment may be used to reduce cylinder overheating, thereby reducing the likelihood of cylinder pre-ignition events. By alternating enrichment with enleanment in a given cylinder over consecutive engine cycles, soot accumulation and spark plug coking in the cylinder can be reduced, thereby reducing the occurrence of cylinder misfires. At the same time, by adjusting the enleanment of some cylinders to match the enrichment of other cylinders, a stoichiometric exhaust gas air-to-fuel ratio may be maintained near a downstream emission control device. As such, this allows abnormal cylinder combustion events to be reduced without degrading exhaust emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.